Although there has been considerable progress in understanding the central neural processing of taste information and the receptor and transduction mechanisms for taste stimuli, including several recent advances from molecular biology, the link between different levels of analysis remains elusive. Studies conducted during the previous funding period have provided a much better understanding of the relationship between receptor mechanisms, central neural responses, and behavior elicited by sodium and nonsodium salts and acids. However, there is little information about such relationships for stimuli that humans classify as sweet or bitter. The studies proposed in this application seek to link taste receptor cell (TRC) activity to the responses of central gustatory neurons and behavior in rats. These experiments use a newly developed method for recording from TRCs in intact gustatory epithelia to examine the distribution of sensitivities to sweet and bitter stimuli and to relate these data to responses to these stimuli in rat brainstem neurons and to stimulus relationships established in rat behavioral experiments. These studies address current questions in gustatory neurobiology and test several underlying hypotheses about the coding of information for stimuli described as sweet or bitter by humans. The first Specific Aim, which will continue a collaborative effort with Dr. Tim Gilbertson, utilizes a newly developed method for recording from TRCs in intact fungiform and palatal epithelia of the rat to investigate the distribution of sensitivities to bitter and sweet stimuli in these cells. These studies test the hypothesis that receptor cells vary in their sensitivities to stimuli within the bitter and sweet categories. In addition, transduction pathways for diverse bitter substances will be investigated in this preparation using pharmacological blockers. The second Specific Aim involves single-neuron recording from the rat NST to investigate the central neural coding of sweet and bitter stimuli. These studies test the hypothesis that the patterns of central neural activity generated via the VIIth nerve can be predicted from the distributions of receptor activity demonstrated in Aim 1. The third Specific Aim employs behavioral methods to generate psychophysical functions in rats to arrays of sweet and bitter stimuli. These functions will be used to derive similarity indices among these stimuli to test the hypothesis that behavioral similarity can be predicted from receptor and brainstem responses recorded in Aims 1 and 2. The overarching goal of these proposed experiments is to determine, in a single species and with the same stimuli, how the activity of individual TRCs contributes to the central neural representation of behaviorally categorized stimuli.